U.S. Pat. No. 4,429,990 issued February 7, 1984 to E. J. Tamary shows a pressure roller fuser of a type presently commercially used to fix toner images to support sheets. An important aspect of that disclosure is a mechanism for applying release liquid to a fusing roller which contacts the toner image. That liquid applying mechanism, commonly called a rotating wick, includes a hollow porous roller, which is supplied with fuser oil internally. The applicator has an inner supply tube with holes in it and is covered by a porous material having a surface of wool or a heat resistant synthetic wicking material. The applicator is rotatable by the fusing roller and thoroughly applies the liquid with minimum wear to the roller. The applicator is movable into and out of engagement with the roller according to a program which prevents excess buildup of oil on the roller which otherwise would stain the image bearing sheet. This general type of structure is relatively inexpensive to manufacture and is effective in applying oil to rollers, both of the type in which oil is applied to one roller and also of the type shown in the patent where the oil is supplied by separate wicks to both rollers.
However, because the wick structure directly contacts the roller and rolls with it, it has a tendency to leave the liquid in a fine pattern consisting of spots of locally excessive liquid. These spots may impede transfer of heat energy from the fusing roller to the toner. With ordinary paper support sheets, the release liquid is readily absorbed and the spots of excess liquid have no adverse effect. Resin based stock, for example transparency stock, does not absorb the liquid and small spots of incomplete fusing can result. These spots are not generally noticed when black toner is used on transparencies. However, even a small amount of incomplete fusing of a color transparency will show up as gray or black spots in a projected color image using that transparency. The present tendency of the art to use very viscous silicone oils as the release liquid has increased this problem of locally excessive liquid.
A number of references show approaches to improving fusing oil control in fusing transparencies; see for example U.S. Pat. Nos. 4,549,803 and 4,593,992. In both of these patents, fixing conditions are changed when the sheet carrying the toner image is a synthetic resin rather than paper. To improve fixing for transparency stock, an obvious solution is to slow the fixing apparatus down or increase the temperature to thereby apply more power to each unit of area to be fused. Obviously, if transparency stock absorbs less release liquid, the amount of liquid applied can be reduced as well. In these two patents this is accomplished by articulating a wick that applies release liquid to an application roller or by control of various spreading and cleaning devices in the system. These patents deal only with the problem of excessive liquid on transparencies in general not with the problem of locally excessive spots of liquid imparted by the wicking structure itself.
The Tamary patent is representative of a number of other patents which show articulating wicks for pressure roller fixers; see for example U.S. Pat. No. 4,008,955; U.S. Pat. No. 4,045,165 and U.S. Pat. No. 4,272,666.